a) Field of the Invention
The present invention relates to a projector-type headlamp having a lamp bulb provided near the inner focus of a concave mirror, and a convex lens refracting forward rays of light emitted from the lamp bulb and reflected at the concave mirror so as to be nearly parallel with each other. More particularly the present invention relates to an improved and novel projector-type headlamp in which the reflected light rays are diffused at a higher rate in a portion near the center of the concave mirror of the light reflector and at a lower rate in the periphery of the concave mirror.
b) Prior-art Statement
The automotive headlamp must be able to illuminate brightly the road surface in front of the car running on a lane while having such a luminous intensity distribution pattern as will not dazzle the driver of a car running on the opposite lane.
For such a luminous intensity distribution as not to dazzle the driver of a car running on the opposite lane, for a simple lens configuration and for a totally compact design, projector-type automotive headlamps have been proposed. FIGS. 1 to 3 show an example projector-type headlamp. FIG. 1 is a schematic plan view of the headlamp, FIG. 2 is a side elevation of the headlamp in FIG. 1, and FIG. 3 is a front view of the headlamp in FIG. 1.
The reference numeral I denotes a concave mirror, and the symbol F denotes a focus of the mirror 1. The reference numeral 2 denotes a lamp bulb of which the filament is located near the focus F. The reference numeral 3 denotes a convex lens having an optical axis Z which is coincident with that of the concave mirror 1.
The line i-j in FIG. 1 indicates the meridional image plane of the convex lens 3, upon which the rays of light emitted from the light source (lamp bulb) and reflected at the concave mirror 1 are incident.
The above-mentioned incident light rays are refracted and emitted forward (rightward in FIGS. 1 and 2) by the convex lens 3. When a screen is disposed near the meridional image plane, an isolux line results from the luminous intensity distribution pattern as shown in FIG. 4. The line H--H is a horizontal line on the screen, and the line V--V is a vertical line on the screen.
As shown in FIGS. 1 to 3, there is provided a shade 4 having a cut line along the meridional image plane. More particularly, the cut line 4a is so formed as to extend downward from the horizontal section i-j of the meridional image plane as shown in FIG. 3. FIG. 4 shows the correspondence between the luminous intensity distribution pattern and the shade 4. As seen, the upper half of the light beam is passed forward. The shade 4 intercepts the majority of the lower half. The rest of the lower half, above the cut line 4a, is allowed to go forward. FIG. 5 is an explanatory drawing schematically showing the light projection pattern by an isolux line on a screen located in front of the headlamp.
The luminous intensity distribution pattern shown in FIG. 5 creates a light beam suitably used when two cars pass each other. However, when a car runs with no car on the opposite lane, the light beam may not be cut by the cut line 4a'. When the shade 4 is omitted from the projector-type headlamp shown in FIGS. 1 to 3, the hatched portion of the light beam in FIG. 4 will not be cut off and thus a luminous intensity distribution pattern shown in FIG. 6 results.
FIGS. 7(A) and 7(B) are explanatory drawings showing in further detail the luminous intensity distribution pattern of the conventional projector-type headlamp. The pattern in FIG. 7(A) is essentially similar to that shown in FIG. 6. In these Figures, the maximum luminosity zone of the pattern (so-called "hot zone") is shown as smudged. The luminosity distribution along the line H--H in FIG. 7(A) is shown in FIG. 7(B).
As seen from FIG. 7(B), the existence of the high-luminosity (hot) zone formed near the center of the luminous intensity distribution pattern will make it difficult for the car driver to discriminate or visually recognize the vehicle, walker or other object in the lower-luminosity zone around the hot zone. Namely, the car driver feels the luminosity in the hot zone too strong to drive the car safely.
Any effective technique to alleviate the driver's feeling that the spotzone luminosity is too strong has not yet been developed, but a technique to suppress the unevenness of the luminous intensity distribution in the hot zone has been proposed in, for example, the Japanese Unexamined Patent Publication No. 01-276502. FIGS. 8 and 9 are drawings for explaining this conventional technique. As seen from FIG. 8, the entire reflecting inner surface of the concave mirror 21 comprises a plurality of vertical, horizontally symmetrical segments a, a'; b, b'; . . . ; i, i'. Each of the segments is formed from a curved cylindrical surface. As shown in FIG. 9, the segments a, b, . . . , i in the right half of the reflecting surface of the concave mirror 21 are so arranged as to form a horizontal section which is a nearly concave arc. The reference symbols a', b', . . . , i' for the segments in the left half are omitted but they are so arranged as to form a similar nearly concave arc.
Such reflecting inner surface of the concave mirror 21 is composed of a plurality of segments formed on the inner surface of an ellipse having a first focus F.sub.1 and second focus F.sub.2. A lamp bulb is disposed nearly on the first focus F.sub.1. In an ordinary conventional projector-type headlamp having the elliptical surface as the reflecting inner surface, the light rays emitted from the lamp bulb located at the first focus F.sub.1 are concentrated at the second focus F.sub.2.
In case of the concave mirror 21 having the reflecting inner surface composed of a plurality of above-mentioned segments; however, the light rays emitted from the lamp bulb located at the first focus F.sub.1 are slightly diffused and concentrated at a position somehow off the focus F.sub.2, that is, between points f.sub.1 and f.sub.2. As a result, the filament image formed at the second focus F.sub.2 will be blurred horizontally.
When projected forward through the convex lens 3 shown in FIG. 9, the blurred image will diverge through a very small angle of .DELTA..theta. (smaller than 1.degree. in this case). As a result, the unevenness of luminous intensity distribution in the hot zone shown in FIG. 7(B) will be suppressed. With the conventional projector-type headlamp using a concave mirror having such a curved cylindrical diffusion surface, however, the driver's feeling that the luminosity in the hot zone formed near the center of the luminous intensity distribution pattern is too strong cannot yet be eliminated.
As shown in FIG. 8, the lamp bulb is located near the point O, the center of the concave mirror 21. Of the reflecting inner surface of the concave mirror, the portion near the axis X--X (for example, portion J) is nearer the light source than the portion far from the axis X--X (for example, portion K), so that the luminous density of the incident light upon the portion near the axis X--X is larger. Therefore, the reflected light at the portion near the axis X--X is contributed primarily to forming of a hot zone.
For these points of view, it is more effective to diffuse the reflected light at the portion near the center of the concave mirror than to diffuse the reflected light at the peripheral portion of the concave mirror, for the driver not to feel the luminosity of the hot zone too strong.